Load Cells and Force Sensors in Robotics

Preface

Load cells and force sensors have been around for many years. However, these mature technologies are continually adapting to the needs of emerging technologies like robotics. Admittedly, robotics isn’t the latest technology either. Robots can be traced back to the early ’60s when automotive production lines began employing them. Over decades, robots have reached into many other industries, including medical technology. It is these new applications that keep robotics in the category of emerging technology.

“Load Cells” vs. “Force Sensors” in Robots

Before discussing the applications of force sensors and load cells in robotics, it is helpful to clarify the distinction between these terms. The term “load cell” primarily describes a device that measures weight. The term “force sensor” mainly refers to a device that measures pressure. Since pressure is nothing more than weight per unit area, the two terms describe the same physics. Typically the robotics field prefers the term “force sensor” while the traditional weight measurement field, those using strain gauges and other means or materials, uses the term “load cell.” The purist can click here to learn all about load cells including a device called a pressure load cell. Meanwhile this resource describes force sensors for robots, including a button load cell sensor. These two sources help illustrate the distinguishing characteristics between load cells and force sensors. All are sensors that fit different applications.

Robotics Industry Trends for Force and Tactile Sensors

In addition to clarifying terms, it is likewise important to understand current trends in robotics. Right now in the industry, Collaborative Robots (Co-Bots) are seen as the future of manufacturing. The reasons are self-evident.

Robotics save both time and money, being available for product throughput 24/7.

Robots are scalable and adaptable resources. They can be used for high production, high volume jobs one day, then scaled down for a specialized low volume job the next day.

Robotics creates jobs. Although robots replace production line workers, human resources must keep them ready to do different jobs in different parts of a factory. The map in Figure 1 shows the deficit of robotics talent in 2018, which are projected to grow [1].

Figure 1

The future installation rate of new robot resources is expected to climb by 12%/yr over the next few years, as the chart in Figure 2 demonstrates [2]. The global robotics and autonomous systems market is expected to grow to $275B by 2025 [3]. The need for load cells and force-sensing technology will grow right along with it.

Figure 2

New Applications for Robotic Force and Tactile Sensors

Most robots today, especially collaborative robots, have force and torque sensing abilities, but its primary use is safety when working close to humans.

Today, new applications of force and tactile sensors are far beyond being a safety net. New levels of precision are the latest innovations in industrial robotics. With modern tech sensors and advances in control software, robots are developing a “feel” for what they touch.

These new levels of precision and accuracy associated with this innovative tech allow collaborative robots to take over many of the more delicate tasks such as:

Gripping

Grinding

Deburring

Polishing

Product assembly

Product packaging

Product testing

A force-sensing technology on the rise for rapid order fulfillment is ‘gripping.’ Gripping tech is expected to be the fastest-growing warehouse automation technology in the US by 2020 [4]. The combination of machine learning software, improvements in force-sensing technology, and easier to program, application-centric solutions are critical to this growth.

Product assembly is another example from the list above made possible by tactile sensing technology. These sensors enable processes that have very close tolerances for clearance around parts, such as inserting pistons into an engine in automobile manufacturing. Currently, robotic vision systems are not capable of providing the precision needed for these jobs. Force-sensing devices provide the highest level of accuracy to find the precise tool center point for use in inserting the piston [5]. Yes, “touch and feel” are exciting concepts for the venerable load cell sensor as they are taking on some very human aspects of getting jobs done.

Innovations in Robotic Load Cell and Force-Sensing Technology

As stated before, innovations in technology are enabling the robotics applications listed above. These are mainly innovations in programming and innovations in materials.

Programming Innovations

Robots have matured to the point where they can adjust to real-world variables through new force-sensing technology and machine learning software. These new collaborative robots don’t require specialized programming. The machine learning algorithms in the software load are self-teaching. A human trains the machine to perform the process merely by dragging the robot arm along with him. The robot will then mimic the motion and learn more from each iteration until it’s doing the job to specification. The accuracy and precision of the force-sensing devices in the robot combined with the machine learning algorithms do all the heavy lifting.

Materials Innovations

Lastly, it is important to mention new applications for materials in talking about force sensors in robotics innovations. Graphene is a material that was first isolated in 2004, and innovators are now widely realizing its value, including it in pressure sensors.

What is graphene? Graphene is a one-atom-thick layer of carbon atoms arranged in a hexagonal lattice and is the building block of graphite (the same stuff in your #2 pencil). Although it is the thinnest material known to man at just one atom thickness, it is very strong (about 200 times stronger than steel) and highly conductive of both heat and electricity [6]. Because of its physical properties, graphene opens the door for the development of nanoscale sensors. These make possible cell-sized robots for medical research, diagnosis, and many more new discoveries over time.

In general, graphene sensors are capacitive or piezoelectric sensors (see An Overview of Load Cells), creating an electric field when pressure is applied. They function with very low power and are far less influenced by temperature than common strain gauge sensors. This public document [7] explains the challenges of fabricating graphene sensors and some advances in the adaptation of the material.

As an example of the versatility of graphene in sensor technology, researchers in 2016 demonstrated a biocompatible pressure sensor made from mixing graphene flakes with cross-linked polysilicone (silly putty) [8].

In the future, the micro size, superior strength and favorable response in unfavorable ambient conditions will bring graphene to the forefront of sensor applications in robotics.

Conclusion

As this article demonstrates, load cells are advancing the field of robotics immeasurably. New programming techniques combined with new materials are enabling high-precision and nanoscale applications. What was once science fiction is now becoming reality.